The present invention generally relates to a differential amplifier used mainly for audio circuits and, more particularly, to a complementary differential amplifier having a pair of differential amplifier circuits operating in a complementary manner.
Complementary differential amplifiers, which are superior to conventional differential amplifiers with a pair of amplifying elements, have recently been used in audio circuits. An example of the conventional complementary differential amplifiers is shown in FIG. 1. The complementary differential amplifier circuit is comprised of a first differential amplifier circuit 3 having NPN type transistors 1 and 2 and a constant current source 7 (current value Ie) and a second differential amplifier circuit 6 having PNP transistors 4 and 5 and a constant current source 8 (current value Ie). Assuming that the collector current of the transistor 1 is Ia, the collector current of the transistor 4 is Ib, and a difference between the two input voltages V1 and V2 is Vi (Vi=V1-V2), its operation characteristics are depicted as shown in FIG. 2. In FIG. 2, a broken line A designates the current Ia of the first differential amplifier 3, a broken line B the current Ib of the second differential amplifier 6, a continuous line C the sum of the currents Ia and Ib. The slopes of the broken lines A and B used for an output current are each given by 1/2R1 where R1 is the resistance value of resistors 9, 10, 11 and 12, and the slope of the continuous line by 1/R1. Further, points P1 and P2 denote the values of the current Ia and Ib in no input signal condition (Vi=0).
In the complementary differential amplifier, when the input voltge Vi exceeds .+-.R1.times.Ie, the output current saturates at a magnitude of Ie or -Ie. Therefore, the dynamic range and the slew rate of the circuit is small and when the circuit is used for the first stage of a negative feedback circuit in such a manner that the feedback signal is applied as the input voltage V2, TIM (transient intermodulation) distortion tends to occur.
To increase the saturation output current, there is proposed a complementary differential amplifier as shown in FIG. 3. In the circuit of FIG. 3, when input voltage Vi (V1-V2)&gt;0, the output current flows through transistors 15 and 16, while when Vi&lt;0, the output current flows through transistors 17 and 18. Therefore, the maximum output current is allowed up to 2Vc/2R2 (Vc: the voltage of each of DC power sources 19; and 20 and R2: the value of resistors 21 to 24). This circuit, however, requires separate constant voltage bias circuits 25 and 26 in the input circuit. Further, those circuits must be precisely matched with each other. Thus, the conventional complementary differential amplifier needs separate bias circuits, resulting in complicated and uneconomical circuit construction.